This invention relates to wet process chemistry (galvanic or electroless) plating cells, and is more particularly directed to a technique that provides an even distribution of electrolyte or plating solution onto and across a substrate to be plated, and which prevents accumulation of bubbles or other plating by-products on the surface of the substrate. The invention is more particularly directed to an improved plating cell for either galvanic or electroless plating in which megasonic energy is applied to the solution in the plating cell. The invention is more specifically directed to a plating cell in which a fluid powered rotary wiper, in combination with the megasonic action of the transducer, ensures efficient and uniform plating, regardless whether the workpieces or substrates are rotated during the plating process.
Electroplating plays a significant role in the production of many rather sophisticated technology products, such as masters and stampers for use in producing digital compact discs or CDs. However, as these products have become more and more sophisticated, the tolerances of the plating process have become narrower and narrower. For example, in a modem CD, impurities or blemishes of one micron or larger can create unacceptable data losses. Current electroplating techniques can result in block error rates of 70, and with higher density recordings, the block error rate can be 90 or higher. Current plans to increase the data density of compact discs are being thwarted by the inability of plating techniques to control blemishes in the plating process.
A number of techniques for electro-depositing or coating on an article face have been described in the patent literature, but none of these achieves the high plating purity and evenness of application that are required for super-high density compact discs.
A recent technique that employs a laminar flow sparger or injection nozzle within the plating bath is described in my recent patent application Ser. No. 08/556,463, filed Nov. 13, 1995, now U.S. Pat. No. 5,597,460. The means described there achieve an even, laminar flow across the face of the substrate during the plating operation. A backwash technique carries the sludge and particulate impurities away from the article to be plated, and produces a flat plated article of high tolerance, such as a high-density compact disc master or stamper.
In the manufacture of compact discs, there is a step that involves the use of a so-called stamper. The stampers are negative discs that are pressed against the material for the final discs to create an impression that becomes the pattern of tracks in the product compact discs.
Stampers are nickel and are electroformed. The stampers are deposited on a substrate that has the data tracks formed on it, and has been provided with a conductive surface, e.g., by sputter coating. Then the substrate is placed into a plating tank. The nickel is introduced in solution into the process cell so that it can be electrochemically adhered onto the substrate surface, using standard electroplating principles. Present industry standards require the stamper to have an extremely high degree of flatness, and where higher density storage is to be achieved, the flatness tolerance for the nickel coating becomes narrower and narrower.
The flow regime for the plating solution within the tank or cell is crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the process vessel, distribution of fluid within the vessel and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it contacts and flows across the substrate in the plating cell.
Present day electroplating cells employ a simple technique to inject fluid into the process vessel or cell. Usually, a simple pipe or tube is used with an open end that supplies the solution into the tank or cell. The solution is forced from the open end of the pipe. This technique is not conducive to producing a flat coating, due to the fact that the liquid is not uniformly distributed across the surface of the workpiece. This technique can create high points and low points in the resulting plated layer, because of localized eddies and turbulences in the flow regime.
In the plating cell as described in said U.S. Pat. 5,597,460, a plating bath contains the electrolyte or plating solution, in which the substrate to be plated is submerged in the solution. A sparger or equivalent injection means introduces the solution into the plating bath and forms a laminar flow of the electrolyte or plating solution across the surface of the substrate to be plated. Adjacent the plating bath is an anode chamber in which anode material is disposed, with the material being contained within an anode basket. In a typical CD-stamper forming process, the anode material is in the form of pellets, chunks or nuggets of nickel, which are consumed during the plating process. A weir separates the plating bath from the anode chamber, and permits the plating solution to spill over its top edge from the plating bath into the anode chamber. The weir is in the form of a semipermeable barrier that permits nickel ions to pass through from the anode chamber into the plating bath, but blocks passage of any particulate matter. A circulation system is coupled to the drain outlet to draw off the solution from the anode chamber, together with any entrained particles, and to feed the solution through a microfilter so that all the particles of microscopic size or greater are removed from the plating solution. Then the filtered solution is returned to the sparger and is re-introduced into the plating cell. In this way a backwash of the plating solution is effected, so that the flow regime of the fluid itself washes any particulates out of the anode chamber in the direction away from the plated article. At the same time, the cleansed and purified solution bathes the plated surface of the substrate as a uniform, laminar flow of solution, thus avoiding high spots or voids during plating. As a result, very high tolerance is achieved, permitting production of compact disks of extreme density without significant error rates.
The flow regime as described in said U.S. Pat. No. 5,597,460 is further improved by the geometry of the well that forms the tank for the plating bath. In that patent the substrate can be positioned on either a fixed or a conventional rotary mount. A conventional cathodic motor rotates the substrate, e.g. at 45-50 RPM. The substrate can be preferably oriented anywhere from vertical to about 45 degrees from vertical. The well has a cylindrical wall that is coaxial with the axis of the substrate. This arrangements avoid corners and dead spaces in the plating cell, where either the rotation of the substrate or the flowing movement of the plating solution might otherwise create turbulences.
A U-tube laminar flow sparger, shaped to fit on the lower wall of the plating bath or plating cell, can be positioned adjacent the base of the weir to flow the solution into the space defined between the substrate and the weir. The sparger's flow holes are directed in parallel to create a uniform, laminar flow of the electrolyte across the planar face of the substrate. The axes of the flow holes in the sparger define the flow direction of the plating solution, i.e., generally upwards and parallel to the face of the plated substrate.
Unfortunately, even with these improvements, the plating is not completely even over the substrate. There is a tendency for hydrogen bubbles to accumulate on the surface of the substrate where electrolytic plating is taking place, and these can interfere with the plating and cause errors in the data on the CD master. Also, with conventional plating there is a tendency for the plated surface to become bowed out, that is, for the plated metal layer to lose its flatness away from the center. Consequently, it is necessary to plate a large margin around the target CD master or stamper, so that center part will have the desired flatness. This necessitates using additional time and materials.
Megasonics have been employed in semiconductor wafer processing, but only in connection with cleaning of the wafers prior to plating or etching. Several megasonic devices have been proposed for this purpose, and some of these have been made the subject of U.S. patents.
Shwartzman et al. U.S. Pat. No. 4,118,649 relates to a transducer assembly for producing acoustic energy at megasonic frequencies, i.e., from about 0.2 MHz to about 5 MHz, and applying the megasonic energy to a cleaning tank. Guldi et al. U.S. Pat. No. 5,520,205 and Bran U.S. Pat. No. 5,365,960 each relate to a megasonic cleaning assembly for cleaning semiconductor wafers in a cleaning tank. The megasonic energy is used to loosen material from the surface of the wafers, and it apparently did not occur to anyone involved with the above-mentioned patents to apply megasonic energy for the opposite purpose, namely, to assist in depositing material on the surface of the wafers.
In a metal plating technique, flow regime for the plating solution within the tank or cell is crucial for successful operation. Flow regime is affected by such factors as tank design, fluid movement within the plating cell, distribution of fluid within the cell and at the zone of introduction of the solution into the vessel, and the uniformity of flow of the fluid as it is contacts and flows across the workpiece. However, optimal sparger design can only achieve a limited increase in flatness of metallization.